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2 InterruptsIn the examples analyzed, the program enters a wait loop in which it repeatedly tests the device status. During this period, the processor is not performing any useful computation. There are many situations where other tasks can be performed while waiting for an I/O device to become ready. To allow this to happen, we can arrange for the I/O device to alert the processor when it becomes ready. It can do so by sending a hardware signal called an interrupt request to the processor. Since the processor is no longer required to continuously poll the status of I/O devices, it can use the waiting period to perform other useful tasks. Indeed, by using interrupts, such waiting periods can ideally be eliminated.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

3 InterruptsConsider a task that requires continuous extensive computations to be performed and the results to be displayed on a display device. The displayed results must be updated every ten seconds. The ten-second intervals can be determined by a simple timer circuit, which generates an appropriate signal. The processor treats the timer circuit as an input device that produces a signal that can be interrogated. If this is done by means of polling, the processor will waste considerable time checking the state of the signal. A better solution is to have the timer circuit raise an interrupt request (IRQ) once every ten seconds. In response, the processor displays the latest results.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

4 InterruptsThe task can be implemented with a program that consists of two routines, COMPUTE and DISPLAY. The processor continuously executes the COMPUTE routine. When it receives an interrupt request from the timer, it suspends the execution of the COMPUTE routine and executes the DISPLAY routine which sends the latest results to the display device. Upon completion of the DISPLAY routine, the processor resumes the execution of the COMPUTE routine. Since the time needed to send the results to the display device is very small compared to the ten-second interval, the processor in effect spends almost all of its time executing the COMPUTE routine.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

6 InterruptsAssume that an interrupt request arrives during execution of instruction i. The processor first completes execution of instruction i. Then, it loads the program counter with the address of the first instruction of the interrupt-service routine. After execution of the interrupt-service routine, the processor returns to instruction i + 1. Therefore, when an interrupt occurs, the current contents of the PC, which point to instruction i + 1, must be put in temporary storage in a known location. A return-from-interrupt instruction at the end of the interrupt-service routine reloads the PC from that temporary storage location, causing execution to resume at instruction i + 1. The return address must be saved either in a designated general-purpose register or on the processor stack.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

7 InterruptsWe should note that as part of handling interrupts, the processor must inform the device that its request has been recognized so that it may remove its interrupt-request signal. This can be accomplished by means of a special control signal, called interrupt acknowledge (INTA), which is sent to the device through the interconnection network. An alternative is to have the transfer of data between the processor and the I/O device interface accomplish the same purpose. The execution of an instruction in the interrupt-service routine that accesses the status or data register in the device interface implicitly informs the device that its interrupt request has been recognized.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

8 InterruptsMost modern processors save only the minimum amount of information needed to maintain the integrity of program execution. This is because the process of saving and restoring registers involves memory transfers that increase the total execution time, and hence represent execution overhead. Saving registers also increases the delay between the time an interrupt request is received and the start of execution of the interrupt-service routine. This delay is called interrupt latency. In some applications, a long interrupt latency is unacceptable. For these reasons, the amount of information saved automatically by the processor when an interrupt request is accepted should be kept to a minimum. Typically, the processor saves only the contents of the program counter (PC) and the processor status register (PS).CENG Spring Dr. Yuriy ALYEKSYEYENKOV

9 Enabling and Disabling InterruptsThe facilities provided in a computer must give the programmer complete control over the events that take place during program execution. The arrival of an interrupt request from an external device causes the processor to suspend the execution of one program and start the execution of another. Because interrupts can arrive at any time, they may alter the sequence of events from that envisaged by the programmer. Hence, the interruption of program execution must be carefully controlled. A fundamental facility found in all computers isthe ability to enable and disable such interruptions as desired.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

10 Enabling and Disabling InterruptsThere are many situations in which the processor should ignore interrupt requests. For instance, the timer circuit should raise interrupt requests only when the COMPUTE routine is being executed. It should be prevented from doing so when some other task is being performed. In another case, it may be necessary to guarantee that a particular sequence of instructions is executed to the end without interruption because the interrupt-service routine may change some of the data used by the instructions in question.For these reasons, some means for enabling and disabling interrupts must be available to the programmer.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

11 Enabling and Disabling InterruptsIt is convenient to be able to enable and disable interrupts at both the processor and I/O device ends. The processor can either accept or ignore interrupt requests. An I/O device can either be allowed to raise interrupt requests or prevented from doing so. A commonly used mechanism to achieve this is to use some control bits in registers that can be accessed by program instructions.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

12 Enabling and Disabling InterruptsThe processor has a status register (PS), which contains information about its current state of operation. Let one bit, IE, of this register be assigned for enabling/disabling interrupts. Then, the programmer can set or clear IE to cause the desired action. When IE = 1, interrupt requests from I/O devices are accepted and serviced by the processor. When IE = 0, the processor simply ignores all interrupt requests from I/O devices.The interface of an I/O device includes a control register that contains the information that governs the mode of operation of the device. One bit in this register may be dedicated to interrupt control. The I/O device is allowed to raise interrupt requests only when this bit is set to 1.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

13 Enabling and Disabling InterruptsLet us now consider the specific case of a single interrupt request from one device. When a device activates the interrupt-request signal, it keeps this signal activated until it learns that the processor has accepted its request. This means that the interrupt-request signal will be active during execution of the interrupt-service routine, perhaps until an instruction is reached that accesses the device in question. It is essential to ensure that this active request signal does not lead to successive interruptions, causing the system to enter an infinite loop from which it cannot recover.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

14 Enabling and Disabling InterruptsA good choice is to have the processor automatically disable interrupts before starting the execution of the interrupt-service routine. The processor saves the contents of the program counter and the processor status register. After saving the contents of the PS register, with the IE bit equal to 1, the processor clears the IE bit in the PS register, thus disabling further interrupts. Then, it begins execution of the interrupt-service routine. When a return-from-interrupt instruction is executed, the saved contents of the PS register are restored, setting the IE bit back to 1. Hence, interrupts are again enabled.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

15 Enabling and Disabling InterruptsBefore proceeding to study more complex aspects of interrupts, let us summarize the sequence of events involved in handling an interrupt request from a single device. Assuming that interrupts are enabled in both the processor and the device, the following is a typical scenario.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

16 Enabling and Disabling Interrupts1. The device raises an interrupt request.2. The processor interrupts the program currently being executed and saves the contents of the PC and PS registers.3. Interrupts are disabled by clearing the IE bit in the PS to 0.4. The action requested by the interrupt is performed by the interrupt-service routine, during which time the device is informed that its request has been recognized (INTA), and in response, it deactivates the interrupt-request signal.5. Upon completion of the interrupt-service routine, the saved contents of the PC and PS registers are restored (enabling interrupts by setting the IE bit to 1), and execution of the interrupted program is resumed.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

17 Handling Multiple DevicesLet us now consider the situation where a number of devices capable of initiating interrupts are connected to the processor. Because these devices are operationally independent, there is no definite order in which they will generate interrupts. For example, device X may request an interrupt while an interrupt caused by device Y is being serviced, or several devices may request interrupts at exactly the same time. This gives rise to a number of questions:CENG Spring Dr. Yuriy ALYEKSYEYENKOV

18 Handling Multiple Devices1. How can the processor determine which device is requesting an interrupt?2. Given that different devices are likely to require different interrupt-service routines, how can the processor obtain the starting address of the appropriate routine in each case?3. Should a device be allowed to interrupt the processor while another interrupt is being serviced?4. How should two or more simultaneous interrupt requests be handled?CENG Spring Dr. Yuriy ALYEKSYEYENKOV

19 Handling Multiple DevicesWhen an interrupt request is received it is necessary to identify the particular device that raised the request. Furthermore, if two devices raise interrupt requests at the same time, it must be possible to break the tie and select one of the two requests for service. When the interrupt-service routine for the selected device has been completed, the second request can be serviced.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

20 Handling Multiple DevicesThe information needed to determine whether a device is requesting an interrupt is available in its status register. When the device raises an interrupt request, it sets to 1 a bit in its status register, which we will call the IRQ bit. The simplest way to identify the interrupting device is to have the interrupt-service routine poll all I/O devices in the system. The first device encountered with its IRQ bit set to 1 is the device that should be serviced. An appropriate subroutine is then called to provide the requested service.The polling scheme is easy to implement. Its main disadvantage is the time spent interrogating the IRQ bits of devices that may not be requesting any service. An alternative approach is to use vectored interrupts.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

21 Vectored InterruptsTo reduce the time involved in the polling process, a device requesting an interrupt may identify itself directly to the processor. Then, the processor can immediately start executing the corresponding interrupt-service routine. The term vectored interrupts refers to interrupt-handling schemes based on this approach.A device requesting an interrupt can identify itself if it has its own interrupt-request signal, or if it can send a special code to the processor through the interconnection network. The processor’s circuits determine the memory address of the required interrupt-service routine. A commonly used scheme is to allocate permanently an area in the memory to hold the addresses of interrupt-service routines.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

22 Vectored InterruptsThese addresses are usually referred to as interrupt vectors, and they are said to constitute the interrupt-vector table. For example, 128 bytes may be allocated to hold a table of 32 interrupt vectors. Typically, the interrupt vector table is in the lowest-address range. The interrupt-service routines may be located anywhere in the memory. When an interrupt request arrives, the information provided by the requesting device is used as a pointer into the interrupt-vector table, and the address in the corresponding interrupt vector is automatically loaded into the program counter.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

23 Interrupt NestingThere was declared, that interrupts should be disabled during the execution of an interrupt-service routine, to ensure that a request from one device will not cause more than one interruption. The same arrangement is often used when several devices are involved, in which case execution of a given interrupt-service routine, once started, always continues to completion before the processor accepts an interrupt request from a second device. Interrupt-service routines are typically short, and the delay they may cause is acceptable for most simple devices.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

24 Interrupt NestingFor some devices, however, a long delay in responding to an interrupt request may lead to erroneous operation. Consider, for example, a computer that keeps track of the time of day using a real-time clock. This is a device that sends interrupt requests to the processor at regular intervals. For each of these requests, the processor executes a short interrupt-service routine to increment a set of counters in the memory that keep track of time in seconds, minutes, and so on. Proper operation requires that the delay in responding to an interrupt request from the real-time clock be small in comparison with the interval between two successive requests. To ensure that this requirement is satisfied in the presence of other interrupting devices, it may be necessary to accept an interrupt request from the clock during the execution of an interrupt-service routine for another device, i.e., to nest interrupts.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

25 Interrupt NestingThis example suggests that I/O devices should be organized in a priority structure. An interrupt request from a high-priority device should be accepted while the processor is servicing a request from a lower-priority device. A multiple-level priority organization means that during execution of an interrupt service routine, interrupt requests will be accepted from some devices but not from others, depending upon the device’s priority. To implement this scheme, we can assign a priority level to the processor that can be changed under program control. The priority level of the processor is the priority of the program that is currently being executed. The processor accepts interrupts only from devices that have priorities higher than its own.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

26 Interrupt NestingAt the time that execution of an interrupt-service routine for some device is started, the priority of the processor is raised to that of the device either automatically or with special instructions. This action disables interrupts from devices that have the same or lower level of priority. However, interrupt requests from higher-priority devices will continue to be accepted. The processor’s priority can be encoded in a few bits of the processor status register.Finally, we should point out that if nested interrupts are allowed, then each interrupt service routine must save on the stack the saved contents of the program counter and the status register. This has to be done before the interrupt-service routine enables nesting by setting the IE bit in the status register to 1.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

27 Simultaneous RequestsWe need to consider the problem of simultaneous arrivals of interrupt requests from two or more devices. The processor must have some means of deciding which request to service first. Polling the status registers of the I/O devices is the simplest such mechanism. In this case, priority is determined by the order in which the devices are polled. When vectored interrupts are used, we must ensure that only one device is selected to send its interrupt vector code. This is done in hardware, by using arbitration circuits.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

28 Controlling I/O Device BehaviorIt is important to ensure that interrupt requests are generated only by those I/O devices that the processor is currently willing to recognize. Hence, we need a mechanism in the interface circuits of individual devices to control whether a device is allowed to interrupt the processor. The control needed is usually provided in the form of an interrupt-enable bit in the device’s interface circuit. I/O devices vary in complexity from simple to quite complex. Simple devices, such as a keyboard, require little in the way of control. Complex devices may have a number of possible modes of operation, which must be controlled. A commonly used approach is to provide a control register in the device interface, which holds the information needed to control the behavior of the device.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

29 Processor Control RegistersIt is important to ensure that interrupt requests are generated only by those I/O devices that the processor is currently willing to recognize. Hence, we need a mechanism in the interface circuits of individual devices to control whether a device is allowed to interrupt the processor. The control needed is usually provided in the form of an interrupt-enable bit in the device’s interface circuit. I/O devices vary in complexity from simple to quite complex. Simple devices, such as a keyboard, require little in the way of control. Complex devices may have a number of possible modes of operation, which must be controlled. A commonly used approach is to provide a control register in the device interface, which holds the information needed to control the behavior of the device.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

30 Processor Control RegistersTimer Display KeyboardThe status register, PS, includes the interrupt-enable bit, IE, in addition to other status information. Recall that the processor will accept interrupts only when this bit is set to 1. The IPS register is used to automatically save the contents of PS when an interrupt request is received and accepted.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

31 Processor Control RegistersThe status register, PS, includes the interrupt-enable bit, IE, in addition to other status information. Recall that the processor will accept interrupts only when this bit is set to 1. The IPS register is used to automatically save the contents of PS when an interrupt request is received and accepted.At the end of the interrupt-service routine, the previous state of the processor is automatically restored by transferring the contents of IPS into PS. Since there is only one register available for storing the previous status information, it becomes necessary to save the contents of IPS on the stack if nested interrupts are allowed.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

32 Processor Control RegistersThe IENABLE register allows the processor to selectively respond to individual I/O devices. A bit may be assigned for each device, as shown in the figure for the keyboard, display, and a timer circuit that we will use in a later example. When a bit is set to 1, the processor will accept interrupt requests from the corresponding device.The IPENDING register indicates the active interrupt requests. This is convenient when multiple devices may raise requests at the same time. Then, a program can decide which interrupt should be serviced first.In a 32-bit processor, the control registers are 32 bits long. For this structure, it is possible to accommodate 32 I/O devices in a straightforward manner.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

33 Processor Control RegistersAssembly-language instructions can refer to processor control registers by using names such as those in Figure. But, these registers cannot be accessed in the same way as the general-purpose registers. They cannot be accessed by arithmetic and logic instructions.Special instructions or special addressing modes may be provided to access the processor control registers. In a RISC-style processor, the special instructions may be of the typeMoveControl R2, PSwhich loads the contents of the program status register into register R2, andMoveControl IENABLE, R3which places the contents of R3 into the IENABLE register. These instructions perform transfers between control and general-purpose registers.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

34 Example of Interrupt ProgramsLet us consider again the task of reading a line of characters typed on a keyboard, storing the characters in the main memory, and displaying them on a display device. We shall show how this task may be performed by using the polling approach to detect when the I/O devices are ready for data transfer. Now, we will use interrupts with the keyboard, but polling with the display.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

35 Example of Interrupt ProgramsWe assume for now that a specific memory location, ILOC, is dedicated for dealing with interrupts, and that it contains the first instruction of the interrupt-service routine. Whenever an interrupt request arrives at the processor, and processor interrupts are enabled, the processor will automatically:• Save the contents of the program counter, either in a processor register that holds the return address or on the processor stack.• Save the contents of the status register PS by transferring them into the IPS register, and clear the IE bit in the PS.• Load the address ILOC into the program counter.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

36 Example of Interrupt ProgramsAssume that in the Main program we wish to read a line from the keyboard and store the characters in successive byte locations in the memory, starting at location LINE. Also, assume that the interrupt-service routine has been loaded in the memory, starting at location ILOC. The Main program has to initialize the interrupt process as follows:1. Load the address LINE into a memory location PNTR. The interrupt-service routine will use this location as a pointer to store the input characters in the memory.2. Enable interrupts in the keyboard interface by setting to 1 the KIE bit in the KBD_CONT register.3. Enable the processor to accept interrupts from the keyboard by setting to 1 the KBD bit in its control register IENABLE.4. Enable the processor to respond to interrupts in general by setting to 1 the IE bit in the processor status register, PS.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

37 Example of Interrupt ProgramsOnce this initialization is completed, typing a character on the keyboard will cause an interrupt request to be generated by the keyboard interface. The program being executed at that time will be interrupted and the interrupt-service routine will be executed. This routine must perform the following tasks:1. Read the input character from the keyboard input data register. This will cause the interface circuit to remove its interrupt request.2. Store the character in the memory location pointed to by PNTR, and increment PNTR.3. Display the character using the polling approach.4. When the end of the line is reached, disable keyboard interrupts and inform the Main program.5. Return from interrupt.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

40 Example of Interrupt ProgramsWhen multiple I/O devices raise interrupt requests, it is necessary to determine which device has requested an interrupt. This can be done in software by checking the information in the IPENDING control register and choosing the interrupt-service routine that should be executed.In the latter example, we used interrupts with the keyboard only. The display device can also use interrupts. Suppose a program needs to display a page of text stored in the memory. This can be done by having the processor send a character whenever the display interface is ready, which may be indicated by an interrupt request. Assume that both the display and the keyboard are used by this program, and that both are enabled to raise interrupt requests. Using the register structure, the initialization of interrupts and the processing of requests can be done as indicated.CENG Spring Dr. Yuriy ALYEKSYEYENKOV

41 Example of Interrupt ProgramsThe Main program must initialize any variables needed by the interrupt-service routines, such as the memory buffer pointers. Then, it enables interrupts in both the keyboard and display interfaces. Next, it enables interrupts in the processor control register IENABLE. Note that the immediate value 6, which is loaded into this register, sets bits KBD and DISP to 1. Finally, the processor is enabled to respond to interrupts in general by setting to 1 the IE bit in the processor status register, PS. Again, we assume that whenever an interrupt request arrives, the processor will automatically save the contents of the program counter (PC) and then load the address ILOC into PC. It will also save the contents of the status register (PS) by transferring them into the IPS register, and disable interrupts.CENG Spring Dr. Yuriy ALYEKSYEYENKOV